Electrical transformers commonly transfer electrical energy from one electrical circuit to another electrical circuit through inductively coupled conductors or coils. Such transformers commonly include one or more conductive coils that are maintained in proximity to one another such that electrical power can be input through a primary or first coil and generates electrical output power in a secondary, adjacent, or second coil. A varying current in the first or primary winding or coil creates a varying magnetic flux in a core and thereby a varying magnetic field in a second winding or coil. The varying magnetic field induces an electromotive force in the secondary coil thereby inducing the electrical output voltage. Such electrical transformers perform many power manipulation processes in many industrial, commercial, and residential applications.
Desired operation of such transformers relies heavily on proper operation of the respective windings associated with the primary and secondary coils. Temperature deviations and thermal localizations associated with either of the primary or secondary windings can adversely affect the ability of the respective coils to conduct the input and/or output electrical power associated with the respective coil. Many such transformers are commonly filled with a dielectric fluid, such as mineral oil, natural or synthetic ester fluids or silicon oil in order to maintain a desired thermal operation of the transformer and to mitigate temperature conditions that can detract from the desired operation of the transformer. Left unaddressed, such deviations can result in failure of the respective coil and/or ultimate failure of the transformer and/or unsuitability of the transformer for its intended purpose.
An accurate model of the transformer's thermal performance is needed to simulate how the transformer will respond in power applications during operation of the transformer. As part of acceptance testing on new units, the temperature of the fluid is measured to demonstrate that the temperature of the fluid does not exceed acceptable limits associated with the intended operation of the transformer. The accuracy of the fluid temperature calculations can vary as a function of the accuracy of the information provided or the accuracy of the measured temperature.
When designing new transformers, engineers utilize theoretical parameters to calculate parameters associated with the thermal performance of the transformer. The various parameters cannot be proved until the transformer is tested. That is, similar to calculating winding and winding hot spot temperatures, transformer dielectric fluid temperature calculations are based on environmental and transformer operating conditions along with the transformer design parameters expressed as equation variables. Comparing measured dielectric fluid temperatures to the theoretical calculated dielectric fluid temperatures can provide confirmation of the equation variables and create more accurate models of the thermal performance of the transformer.
Resistance thermometers, also called resistance temperature detectors or resistive thermal devices (RTD's), are sensors that measure temperature by correlating the resistance of the RTD element with temperature. RTD's are commonly used to measure transformer temperatures. Such RTD's can be provided in both wet and dry configurations. In a wet application, the RTD can be directly exposed to the dielectric fluid internal to the housing of the transformer. Some transformer constructions however will not tolerate wet RTD temperature assessments. Transformer users may also elect to use a dry application, to eliminate the risk of dielectric fluid leaks that may occur when using a wet application. In dry applications, the RTD is disposed in a well that is isolated from the fluid flow or secured, via a magnetic or more permanent interaction, to the outside surface of the transformer.
Temperature measurements associated with the dry implementation of such RTD's can be substantially different than actual fluid temperatures due to various thermodynamic factors associated with the communication of heat from the fluid both through the housing and from the housing to atmosphere. That is, the temperature difference between the internal dielectric fluid temperature and an external tank wall measured temperature can be affected by each of the three methods of heat transfer—convection of the heat from the transformer dielectric fluid to the inner tank wall, conduction of heat through the tank wall, and radiation from the outside tank wall to the outside air. This deviation between such measured and actual temperatures is further explained below with respect to one of the plot lines shown in 4.
Ambient testing conditions and ambient operational conditions can also affect the ability to accurately estimate actual conditions associated with operation of the transformer in an intended environment. That is, environmental conditions such as sunlight and relative humidity can also affect the thermal performance associated with operation of a transformer. These environmental variables can also contribute to deviations between actual and measured values that are not otherwise accommodated or addressed in estimating the operational performance of a given transformer. Accordingly, there is a need for a system and method of more accurately estimating the operating temperature of the fluid of a transformer from measured temperatures external to the cavity of the transformer.
Similar considerations can also be addressed in the design, construction, testing, and operation of other devices such as circuit breakers, reactors, or other fluid filled devices where it would be advantageous to be able to estimate the temperature of the internal fluid from parameters attained at locations external to the fluid volume. Such a methodology provides for convenient user interaction with the measuring devices and can yield simplified device constructions by limiting the through holes and connectivity requirements associated with internally positioned monitoring or assessment components.
One aspect of the invention contemplates a method of estimating the temperature of a fluid within a housing having a wall. The method includes measuring a temperature of a wall of a housing and measuring an ambient temperature around the housing. The method estimates a temperature of a fluid within the housing using the measured temperature of the wall and the measured ambient temperature.
Another aspect of the invention contemplates a method of estimating the temperature of a fluid contained within a housing defined by a wall—such as the dielectric fluid within a housing of an electrical device. The method includes measuring a temperature of the wall of the housing, measuring an ambient air temperature around the housing; and measuring a relative humidity of air around the housing. The method estimates a temperature of a fluid within the housing as a function of the measured temperature of the wall, the measured ambient air temperature, and the measured relative humidity.
A further aspect of the invention contemplates a system for estimating a temperature of a fluid of a transformer. The system includes a first sensor that is fluidly isolated from the fluid contained in a housing and configured to measure a temperature of the housing of the transformer. A second sensor is configured to measure an ambient temperature around the housing and a controller is connected to the first and second sensors and configured to estimate a temperature of the fluid contained in the housing.
Another aspect of the invention contemplates determining or calculating a heat transfer property of one or more of convection heat transfer between the fluid within the housing and the housing, conduction heat transfer through the wall of the housing, and radiation heat transfer from the wall to atmosphere around the housing and manipulating the estimated temperature of the fluid as a function of the heat transfer property associated with a transformer.
These and various other aspects, features, and advantages of the present invention will be made apparent from the following detailed description and the drawings.